Steam turbine rotors operating at low temperatures in wet steam can suffer from the problem of stress corrosion cracking (SCC). The problem has been particularly associated with rotors having shrunk-on discs but it also occurs in monobloc and welded rotors. The main influences for SCC initiation and propagation are (i) the yield strength (often defined as the indication of the maximum stress that can be developed in a material without causing significant plastic deformation) of the rotor material, (ii) the operating stress of the rotor, and (iii) the temperature and operating environment of the rotor. For a given temperature, the stress required for SCC initiation in steam turbine rotor steel increases as the yield strength of the material decreases. Similarly, for a given yield strength, the stress required for SCC initiation decreases as the temperature increases. It is therefore possible to produce a family of threshold curves for any particular steam turbine rotor that interrelate the yield strength of the material, component stress and the operating temperature. If the rotor of a particular yield strength is operated at stresses and/or temperatures that exceed its particular threshold curve then it is considered vulnerable to SCC.
The known practice in the steam turbine industry is to heat treat monobloc and welded rotors for low temperature application in wet steam so that they have uniform yield strength throughout. However, the strength needed to support the moving blades of the last, large diameter turbine stage is usually significantly greater than the strength needed to support the moving blades of the earlier, smaller diameter turbine stages located upstream. Not only does this mean that the parts of the rotor that support the earlier turbine stages are “over-engineered” in terms of their yield strength, it is also the case that these parts can be more vulnerable to SCC because they are operating at a higher temperature and in a harsher wet steam environment. As a result, the parts of the conventional monobloc and welded rotors that support the moving blades of the earlier turbine stages can be particularly vulnerable to SCC initiation and propagation.